The C2-symmetric diester 1 was identified by random screening as a novel inhibitor of HIV-1 proteinase. This led to the preparation of a series of related more potent amides from readily accessible penicillins. Many of the compounds showed potent antiviral activity in HIV-1-infected MT-4 cells and an ability to inhibit syncytia formation in infected C8166 cells, with no evidence of cytotoxicity. The compounds showed no activity against other aspartyl proteinases (renin, pepsin, and cathepsin D). Structure-activity relationships support a symmetrical interaction with the enzyme. Pharmacokinetic evaluation of the ethylamide 3 revealed it was subject to rapid plasma clearance and had low oral bioavailability.